Michael J. Thun, MD, vice president emeritus of epidemiology and surveillance research for the American Cancer Society, and two colleagues reached this conclusion in a paper published in Nature Reviews Clinical Oncology (2012;9:259-267). They noted that although daily aspirin use has been convincingly shown to reduce the risk of colorectal cancer and recurrence of adenomatous polyps, these benefits alone do not outweigh harms from aspirin-induced bleeding in average-risk populations. However, “The accumulating data from randomized clinical trials provide an exciting opportunity to reconsider the potential role of aspirin in cancer prevention,” wrote the review authors.

Although the exact magnitude of the benefits of aspirin in cancer prevention have yet to be determined, even a 10% reduction in overall cancer incidence beginning during the first 10 years of aspirin treatment could tip the balance of benefits and risks favorably in average-risk populations.

Thun and associates described how recently published secondary analyses of cardiovascular trials have provided the first randomized evidence that daily aspirin use may also reduce the incidence of all cancers combined, even at low doses of 75 mg to 100 mg daily. The team found that in six primary-prevention trials of daily low-dose aspirin, randomization to aspirin treatment was associated with an approximate 20% reduction in overall cancer incidence 3 to 5 years later, and a 30% reduction more than 5 years later. Cancer mortality was also reduced more than 5 years after the start of aspirin use in analyses that included 34 trials of daily aspirin at various doses. The observed benefit did not increase with daily aspirin doses higher than 75 mg to100 mg.

Notably, these meta-analyses excluded results from the Women's Health Study, in which participants took 100 mg of aspirin every other day for 10 years. That trial reported no reduction in cancer incidence or mortality.